Lindsay Lohan has a secret 13-year-old half-sister she has never met, the actress' father has revealed.

Michael Lohan has confessed he fathered a child with another woman while separated from the star's mother Dina.

He claims he only found out about his secret daughter years after she was born, and has been exchanging correspondence with her mother over the past year.

Lohan Sr. tells Ok! magazine, "Years later (the woman) contacted me, convincing me that I was the only person she was with and that she had my child."

Ok! claims to have seen letters sent to the girl's mother, in which he compares her to Lindsay. The mother claims Michael has called frequently, but never visited or provided financial support for the girl. And she tells the publication it's time for Michael to take responsibility.

Michael and Dina have four children together; Lindsay, 22; Michael Jr, 20; Ali, 14, and Cody, 12. The couple married in 1985 and divorced in 2007.